I made the transition from sail to power (40' Silverton aftcabin) 6-7 years ago. Needless to say, paying for fuel
was a nasty experience, one we accepted for the convenience power provided. But we thought we were flying in the sailboat at 6K so going slow is enjoyable; half the fun is getting there!
OK, with that stated I checked with Borg-Warner for problems free wheeling their Velvet transmission
at slow speeds. I was told by them there is no problem free wheeling the transmissions in the 7-8K range.
Since then, I operate only ONE of my 454s at a time except when docking
or when absolutely necessary. And that operating engine
runs about 1500 or so RPMs for about 7K SOW. Fuel consumption
is around 1.4-1.6 M/gallon or about 10G/hour.
hold 300 gallons and most years 1 complete fill gets me through the season.